When handling combustible liquids and gases the probability of a fire is so high and the expansion of the fire so rapid that the optical detection technique is the only one which is fast enough to be of use. The fire protection industry has witnessed several attempts at an acceptable optical flame or explosion detection system. However, with each of the techniques used in the past, there have been problems with either inadequate sensitivity or alarm signals from radiation sources other than fire.
Historically, the most effective detection has been with ultra-violet optical detectors. However, these devices are sensitive to arc-welding, lightning and X-rays and thus are plagued with false alarms.
Infra red devices have been used as well. The most successful have been the dual channel devices which compare the emissions at 4.3 microns, characteristic of CO.sub.2 and CO, the by-products of hydrocarbon fires, and 3.8 microns which is a black body emission band. These devices have been limited by the inherent sensitivity of the photocells used. Efforts have been made to improve the performance of the infra-red devices with signal processing circuitry which has been used to superimpose restrictions on the signals to eliminate the possibility of alarm signals due to spurious infra-red emitters.
Various inventions exist which approach the resolution of the flase-alarm problem, but none provide an adequate circuit topology to provide sufficient sensitivity without exposure to false alarms. Hertzberg et al, U.S. Pat. No. 3,859,520, described an explosion detection system which is designed to detect methane explosions. This invention uses three optical channels, one centered on 4.4 microns, and the other two at the sidebands of this band. The idea he presents is that one can use the sidebands as references in a bridge or comparator circuit. When the bridge or comparator circuit detects an imbalance, the alarm output is given. This technique will work where the fuel is limited largely to pure methane. However, real world fires and explosions seldom take place in a manner such that the spectral peak that Hertzberg describes actually exists. In real fires there is broadband, black body radiation superimposed upon the radiation emitted during the oxidation of free radicals and the combustion intermediates which contain more than one carbon to carbon bond. This extraneous radiation makes the simple comparator circuit or bridge circuit ineffective for the detection of the ratios which Hertzberg describes.
Nakauchi, U.S. Pat. No. 4,160,163, describes an invention wherein two channels of optical radiation are converted to electronic signals and the ratio of the two signals are calculated. If the ratio exceeds a predetermined level, the signal is supplied to a subtraction circuit which processes the ratio signal with the 4.4 micron signal and a portion of the 3.8 micron signal. This invention superimposes upon the signals generated by the two bands of radiation an arithmetic process to reduce the exposure to false alarms. The level detection circuit and the arithmetic division and subtraction circuits reduce the sensitivity of the detector to the point where it becomes impractical as a fire detector.
In U.S. Pat. No. 4,220,857, Bright, the inventor, requires the signal from each channel of incident radiation to excede a predetermined level as protection against false alarms. In doing so the detector requires a larger amount of radiation than that which is necessary to satisfy the differential or ratio circuit in order to contribute to stability. Thus its sensitivity is reduced. Bright tried to improve this by reducing the noise level on the input channels with "phase sensitive demodulators". However, the major source of noise is internally generated random electron noise which is not reduced by this circuit. Consequently, even this device provides less sensitivity to a fire than the traditional ultra-violet vacuum photo diode used with competitive u.v. detection systems.
Cinzori, U.S. Pat. No. 3,931,521, and Mc Menamin, U.S. Pat. No. 3,665,440 also have used the technique of using two channels of radiation to detect a fire. However, these inventions use emissions which are very different in wavelength and without signal processing circuitry to prevent false alarms. These inventions are less proficient than those already cited.
It is therefore a primary object of this invention to provide a dual channel infra red detection system with both a high degree of sensitivity to various fire conditions and immunity to false alarms.
It is yet another object of this invention to provide a detection system which can detect both flickering and flash fires with a high degree of sensitivity, while still retaining an immunity to false alarms.
It is still another object of this invention to provide a detection system which includes circuitry to prevent overloading and subsequent false alarms due to direct exposure to sunlight.